1. Technical Field
The invention is related to a plasma reactor for processing a workpiece such as a semiconductor wafer or insulating substrate wherein etch selectivity is enhanced by scavenging etchant species from the plasma, and more particularly to such a reactor wherein the scavenging process is conducted outside the processing chamber of the reactor.
2. Background Art
A plasma reactor may be employed to perform various processes on a semiconductor wafer in microelectronic fabrication. The wafer is placed inside a vacuum chamber of the reactor and process gases, including etchant gases, are introduced into the chamber. The gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Depending upon the composition of the gases from which the plasma is formed, the plasma may be employed to etch a particular material from the wafer or may be employed to deposit a thin film layer of material onto the wafer.
An important factor associated with using a plasma reactor for etching is the etch selectivity. The term etch selectivity refers to the ratio of etch rates of two different materials on a workpiece undergoing etching in the plasma reactor. In one common scenario, it is desired that oxygen-containing materials on a workpiece be etched much faster than an overlying mask formed of photoresist or so-called hardmask material (e.g. SiO2 or Si3N4). Additionally, it is often desired that the oxygen-containing materials be etched much faster than non-oxygen-containing materials of the workpiece. These comparative etch rate relationships are referred to as a high oxide-to-mask and oxide-to-xe2x80x9cnonoxidexe2x80x9d selectivity, respectively. The desirability of this high selectivity will be explained using the example of etching a contact opening through a dielectric layer, such as silicon dioxide (SiO2), to an underlying polysilicon conductor layer and/or to a silicon substrate of a semiconductor wafer. A layer of mask material is formed over the surface of the silicon dioxide layer prior to the etching process in those areas that are not to be etched. Accordingly, there is no mask formed in the area where the contact opening is to be etched. The desired result of the etching process is to quickly etch through the silicon dioxide layer where the contact opening is to be formed, but not to significantly etch the surrounding mask, or the polysilicon or silicon material (or other non-oxygen-containing material such as silicon nitride) underlying the silicon dioxide layer. Thus, high oxide-to-mask and oxide-to-silicon etch selectivities are desired. For a silicon oxide etch process, process gases including an etchant such as fluorine-containing gases are introduced into the chamber. The fluorine-containing gases freely dissociate under typical plasma conditions so much that not only is the silicon oxide layer etched but the mask and the eventually exposed underlying polysilicon or silicon materials are also etched to an unacceptable degree. Thus, without taking steps to ameliorate the effect of excess fluorine-containing etchant species in the plasma on the mask and non-oxide layers of the wafer, a less than desirable etch selectivity results. In fact, if the selectivity is low enough a so-called xe2x80x9cpunch throughxe2x80x9d condition can result wherein the mask layer or a non-oxide layer is etched through causing damage to the device being formed on the wafer. Similar problems related to excess etchant species in the plasma occur in other etch processes as well. For example, polysilicon and silicide (gate) etch processes, or metal etch processes, are subject to degraded selectivity in the presence of excess etchant species.
One method of dealing with the excess of etchant species in the plasma is to introduce a substance that combines with some of the etchant species to form non-etching substances. This process is typically referred to as xe2x80x9cscavengingxe2x80x9d. Ideally, just enough of the etchant species is scavenged from the plasma to increase the selectivity without reducing the etching rate of the material being etched to an unacceptable degree. For example, in the previously-described silicon dioxide etch process, fluorine etchant species are scavenged from the plasma typically by introducing silicon to form the non-etching by-product SiF4. This silicon can be introduced as a component of a gas, or via a solid silicon-containing structure such as one containing pure silicon, polysilicon, silicon carbide (SiC), or a silicon-based dielectric. In the case where a solid silicon-containing source is employed, the source can form a part of the reactor chamber ceiling and/or walls, or it can be a separate piece held within the chamber. Typically, the temperature of the solid silicon-containing source is controlled to prevent it from being covered with deposits comprised of etch by-products or a polymer film (as will be more fully discussed later), and additionally to permit silicon to be more easily removed from the source by the plasma in desired quantities. An RF bias potential is also often applied to a solid silicon-containing source in conjunction with controlling the temperature for the same reasons.
However, in some etching processes the selectivity cannot be increased to satisfactory levels without unacceptably reducing the etch rate of the material being etched from the workpiece. In these situations it is known to introduce a substance into the plasma which causes a protective, etch-resistant layer to deposit on the workpiece materials that are not to be etched, while not depositing on the material to be etch to any significant degree. For example, in the aforementioned silicon dioxide etch process, it is known that the oxide-to-mask and oxide-to-silicon etch selectivity is enhanced by a polymer film that forms more readily over the mask, silicon, polysilicon, and other non-oxygen-containing layers than over silicon dioxide (or other oxygen-containing materials). The polymer resists etching by the fluorine etchant species, thereby increasing the aforementioned selectivity. One common method of forming such a selectivity-enhancing polymer film is to employ a fluorocarbon or fluoro-hydrocarbon gas (e.g., ethyl hexafluoride (C2F6) or trifluoromethane (CHF3)) as the fluorine-containing portion of the process gas. Some of the fluorine-containing species in the plasma are consumed in etching the silicon dioxide layer on the wafer. Other species form a polymer layer on the surface of the wafer. This polymer forms more rapidly and strongly on any exposed non-oxygen-containing surface, such as the mask, silicon or polysilicon surfaces, than on the oxygen-containing surfaces such as the silicon dioxide. In this way the non-oxygen-containing surfaces are protected from the action of the fluorine etching species and the etch selectivity for those surfaces is enhanced. The etch resistance of the polymer can be further strengthened by increasing the proportion of carbon in the polymer relative to fluorine. Typically, the previously-described fluorine scavenging process is employed to reduce the amount of free fluorine in the plasma, thereby resulting in an increase in the carbon content of the polymer.
It is evident from the foregoing description that the scavenging process plays key role in producing a desired etch selectivity in most plasma-enhanced etching processes, including those relying on the formation of a protective film such as the carbon-fluorine polymer employed in silicon oxide etch procedures. However, current etchant species scavenging processes have drawbacks. For example, in the case of a solid silicon scavenging source, one problem is that the rate of removal of silicon from the source required to achieve the necessary decrease in the free fluorine etchant species population of the plasma is so great that the source is rapidly consumed and the consequent need to idle the plasma reactor to replace the source exacts a price in loss of productivity and increase costs. In addition, the size of a modern plasma reactor chamber dictates that the solid silicon source, whether it be integrated into the ceiling and/or walls of the chamber, or a separate piece supported within the chamber, be relatively large so that the scavenging process is uniform across the width of the plasma. This presents a problem as it is difficult to manufacture and control the purity of large silicon structures. As a result, these structures are expensive. Further, this problem is likely to become even worse in view of the current trend to increase the size of the reactor chamber to accommodate ever larger workpieces. The larger reactor chambers will require even bigger silicon sources with a corresponding increase in price.
Another problem with current scavenging processes concerns the devices required to control the temperature of a solid scavenging material source. Typically, the temperature control devices are integrated with the source to reduce the time it takes to change its temperature, thereby ensuring the temperature can be carefully controlled throughout the etch process. This need to integrate portions of the temperature control device into the source itself complicates the structure further, thereby making it even more difficult to manufacture and more expensive. In addition, the larger the source, the more elaborate the temperature control device has to be in order to ensure a precise control of the source""s temperature. Given the aforementioned trend toward up-sizing the reactor chambers, the cost of these consumable scavenging sources may become exorbitant.
An even greater problem with current scavenging processes is that the process parameters, such as the RF power level or chamber temperature, which lead to optimizing etching of the workpiece are not typically those that will maximize selectivity. For example, it is known that increasing the RF power input into the chamber can boost the etch rate. However, this same increase in power also tends to increase the concentration of free etchant species in the plasma which can lead to an undesirable lowering of the oxide-to-mask or oxide-to-nonoxide selectivity. Thus, there is an troublesome tradeoff between the etch rate and selectivity.
Accordingly, there is a need for a plasma reactor design and method of scavenging etchant species from the plasma that does not require the use of large, expensive, scavenging source structures within the reactor""s processing chamber which require costly and frequent replacement. Further, there is a need for such a reactor design and scavenging method that decouples the control of etch selectivity from the control of etch performance, thereby eliminating the undesirable tradeoff between these etch process factors.
The stated needs are fulfilled by an apparatus and method for scavenging etchant species from a plasma formed of etchant gas in a separate scavenging chamber prior to the etchant gas being introduced into the primary processing chamber of the reactor. Scavenging etchant species from the etchant gas prior to feeding it into the primary chamber xe2x80x9cloadsxe2x80x9d the gas with relatively stable, non-etching, etch by-products formed in part from what would have otherwise become etchant species in the plasma created within the primary processing chamber and in part from the material used as a scavenging source in the scavenging chamber. In this way, the concentration of etchant species in the plasma of the primary processing chamber is reduced and the concentration of etch by-products in the plasma is increased, ideally to levels that maximize the oxide-to-mask and oxide-to-nonoxide etch selectivity of the etch process being performed in the reactor. The net result of this method of scavenging etchant species is to eliminate the need for any type of scavenging source structure inside the primary processing chamber of the reactor. Thus, the expense of these large primary chamber scavenging source structures is avoided, as is the cost associated with opening the primary chamber to replace the source.
The preferred apparatus is a plasma reactor that in addition to its primary processing chamber includes at least one scavenging chamber. Each scavenging chamber is connected at an inlet thereof to an etchant gas source and at an outlet thereof to a gas distribution device of the primary processing chamber. Each scavenging chamber has a radiation applicator capable of irradiating the interior of the scavenging chamber and creating a plasma from etchant gas flowing therethrough from the etchant gas source to the gas distribution apparatus of the primary processing chamber. The applicator can be of the type that uses either an inductive discharge, capacitive discharge, or microwave discharge to irradiate the interior of the scavenging chamber and ignite the plasma. An etchant species scavenging source is also included within the scavenging chamber. This source is capable of providing scavenging material that interacts with the plasma to scavenge etchant species created by the dissociation of the etchant gas in the plasma to form non-etching by-products comprised of substances from both the etchant species and the scavenging source.
The etchant species scavenging source is made of a material that will modify the etchant gas in a way that decreases the etch rate of a target material of a workpiece undergoing etch processing in the primary processing chamber of the plasma reactor, thereby increasing the selectivity for the target material. For example, the scavenging source material would preferably be of a type that scavenges the kind of etchant species from the plasma formed within the scavenging chamber and produces the kind of etch by-products that results in the aforementioned lowering of the etch rate of the target material during processing. If practical, the scavenging source could be made of the target material itself to obtain the desired results.
The etchant species scavenging source is also preferably solid and at least partially made of a solid scavenging material. If so, it is also preferred that the scavenging chamber have a removable lid covering an access opening in the chamber. The scavenging source is sized so as to facilitate its being installed into or removed from the chamber through the access opening. Additionally, if the source material is of a type that is expensive and difficult to form into relatively large structures, it is preferred that the source be as small as possible while still being capable of scavenging sufficient etchant species to create a desired concentration of etchant species and etchant by-products in the plasma formed within the primary processing chamber. The power applied to the radiation applicator can be increased and/or the flow rate of the etchant gas through the scavenging chamber can be decreased in order to increase the scavenging capability of the scavenging source, thereby allowing a smaller source to be used in the chamber. A solid source can also incorporate a temperature control apparatus if desired. The temperature control apparatus is capable of controlling the temperature of the scavenging source. Controlling the source temperature provides yet another way of ensuring the scavenging of sufficient etchant species to create a desired concentration of etchant species and etchant by-products in the plasma.
The amount of scavenging, and so the selectivity exhibited in the primary processing chamber, can further be actively controlled via the aforementioned methods. Namely, the power input to the scavenging chamber via the radiation applicator can be adjusted to control the amount of scavenging in the etchant gas flowing through the scavenging chamber. Likewise, the flow rate of the etchant gas through the scavenging chamber can be adjusted to control the scavenging. And finally, the temperature of the scavenging source can be adjusted (if a temperature control apparatus is incorporated) to control the amount of scavenging. It is noted that the above-described control methods allow the selectivity exhibited during etch processing in the primary chamber to be for the most part independently determined regardless of the processing parameters employed in the primary chamber to optimize etch performance. The selectivity-determining scavenging has already occurred before the etchant gas even reaches the primary chamber. Thus, the control of selectivity has truly been decoupled from the control of etch performance using the methods of the present invention, and so the aforementioned tradeoff eliminated.
Alternately, the etchant species scavenging source can be a gaseous scavenging material introduced into the scavenging chamber. Preferably, the gaseous scavenging material would be introduced in a sufficient quantity to create a desired concentration of etchant species and etchant by-products in a plasma formed within the primary processing chamber.
As stated previously, there is at least one scavenging chamber connected to the primary processing chamber of the plasma reactor in accordance with the present invention. However, employing. multiple scavenging chamber can be particularly advantageous. For example, it is often desired to increase the selectivity to more than one material on the workpiece. These separate materials may require that a different etchant species to be scavenged and/or different etching by-products to be loaded into the etchant gas in order to optimize the selectivity. If so, employing multiple scavenging chambers will allow for a separate tailoring of the selectivity of the materials. Essentially, the scavenging material of the source disposed in each scavenging chamber is chosen to scavenge the type of etchant species and produce the type of etch by-product necessary to create the desired selectivity for a particular material of interest on the workpiece. If two such materials are of interest, two scavenging chambers are employed. If it is desired to increase the selectivity for three such materials on the workpiece, three scavenging chambers are employed, and so on.
Further, the above-described scavenging chambers can be employed, as is or in a modified form, as excitation chambers to solve another problem typical of etch processing in a plasma reactor. The process parameters promoting optimal etch conditions (such as a fast etch rate) within the primary processing chamber are also not necessarily conducive to optimizing certain other process factors, exclusive of selectivity. For example, inert gases such as argon (Ar) and helium (He) are often excited in the plasma of the primary processing chamber of a plasma reactor for advantageous effect. However, the process parameters that promote optimal etch performance in the primary chamber, such as the RF power level, are not always conducive to effectively excite these inert gas components of the processing gas. The problem can be resolved by employing a separate excitation chamber outside of the primary processing chamber that excites gases at optimal conditions and feeds the modified gases into the primary chamber. The previously-described scavenging chamber can readily act as such an excitation chamber, the only caveat being that the scavenging source may adversely interact with the gas being excited in some cases. However, as the source is removable from the scavenging chamber, it can simply be removed if such an adverse interaction would occur. Alternatively, one or more dedicated excitation chambers, identical to the previously described scavenging chamber but not capable of supporting a scavenging source, could be incorporated into the reactor, if desired.
In addition to the just described benefits, other objectives and advantages of the present invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.